1. Field of the Invention
The present invention relates to an optical writing device, an image forming apparatus, an optical writing method, and a computer-readable recording medium.
2. Description of the Related Art
Recently, electronization of information has been encouraged, and image processing apparatuses such as printers or facsimiles used for outputting electronic information and scanners used for electronizing documents play essential roles accordingly. Such image processing apparatuses are often configured as multifunction peripherals (MFP) that can be used as a printer, a facsimile, a scanner, and a copying machine, by being provided with an image capturing function, an image forming function, and a communication function, for example.
Among such image processing apparatuses, electrophotographic image forming apparatuses are widely used as image forming apparatuses for outputting electronic documents. An electrophotographic image forming apparatus outputs an electronic document to a paper sheet by exposing the photosensitive element to form an electrostatic latent image, developing the electrostatic latent image into a toner image with a developer such as toner, and transferring the toner image onto the sheet.
As the apparatus ages, a density variation occurs in the toner image formed by development because of various causes. Possible causes include a variation in the conditions in which the photosensitive element is exposed because of wearing of the photosensitive element or deterioration in the light source, and in the conditions in which the photosensitive element is charged because of deterioration in the photosensitive element.
As methods for correcting the density variation caused by aging of the apparatus, besides adjustments of the exposure conditions or the charging conditions, image processing is also possible. This method for correcting density through image processing is called dithering. Dithering is a method that enables image density to be changed without causing any negative impact on the image, by “thinning out” some pixels dispersed across an image to be output, and replacing these pixels with pixels without any color.
Known as a method for realizing a density variation correction through density adjustment with dithering is outputting a plurality of dithering patterns each having a different density at a given interval, reading the patterns to obtain density, and selecting a dithering pattern to be applied to perform density adjustment on the basis of the obtained density. For example, Japanese Patent Application Laid-open No. 2004-336161 describes a method including preparing two different tone scales to be used for dithering and associated with each other, generating a pattern using a dither with a smaller number of tone levels, and reflecting correction information received from a sensor to a dither with a larger number of tone levels in accordance with the association.
Generally, in an image forming apparatus, an image processing unit that generates raster image processor (RIP) data, that is, pixel data of the image to be output, and an engine unit that actually makes an image formation output based on the pixel data are structured as separate units. Therefore, the image processing unit and the engine unit are designed separately. Therefore, interoperability needs to be considered upon designing the image processing unit and the engine unit.
In a method in which the exposure conditions or the charging conditions are adjusted, because the method can be performed within the engine unit, the interoperability with an image processing unit does not need to be considered. By contrast, in the above-described method involving dithering, it is the image processing unit that typically applies dithering to an image, and thus the interoperability between the engine unit and the image processing unit needs to be considered.
To adjust density using dithering, a plurality of dither patterns each having a different density are output as mentioned earlier. Therefore, the resolution of the dithering patterns needs to be matched to the resolution of the engine unit. Hence, to design an image processing unit that is interoperable with different types of engine units, dithering patterns in different resolutions are required. This results in an increase in a required memory capacity in the image processing unit. Furthermore, even if dithering patterns in different resolutions are prepared, only used is the dithering pattern supporting the resolution of the engine unit connected to the image processing unit, and most of the memory capacity is wasted.
Furthermore, when the method disclosed in Japanese Patent Application Laid-open No. 2004-336161 is used, the memory capacity can be reduced by establishing an association between the dither with a smaller number of tone levels and the dither with a larger number of tone levels. However, because the pattern is formed using the dither with a smaller number of tone levels, the resolution of the correction is limited. Accordingly, highly precise correction cannot be provided.
Therefore, there is a need for an optical writing device capable of enabling highly precise density correction through dithering without increasing the complexity of structures of an image processing unit and an engine unit included in an image forming apparatus.